1. Field of the Invention
The present invention relates to hydraulic methods and apparatus for the remote hydraulic control of a device, such as an underwater valve placed in a petroleum well. The invention uses only hydraulic techniques and the control is rapid.
2 The Prior Art
It is known that in petroleum wells, valves known as safety valves are placed at given locations of the well and are designed to close the well if necessary so as to avoid a blowout. Thus, for offshore wells, a blowout preventer is placed on the well-head at sea floor level. In addition, when offshore production tests are carried out from a floating platform, a valve or a set of valves is placed removably near the blowout preventer in the production string. Hence, if it is necessary to abandon the well temporarily, for example as a result of a storm, the valves are closed by a control from the surface and the part of the production string located above the valves is disconnected and brought up to the platform which is then no longer connected to the well. These valves are generally controlled hydraulically from the surface. To accomplish this, hydraulic lines connect the valves to be controlled to a hydraulic fluid source located on the surface of the platform. These lines are advantageously flexible, thereby allowing them to be handled easily and making it possible to place the lines in the well with the lines already connected to the valves, the lines and the valves being installed together. These devices operate satisfactorily when the well-head is not at too great a depth, i.e. when the length of the flexible lines is not too long and, in practice, does not exceed about 300 meters. Beyond this length, the response time of the device, i.e. the time required for opening or closing the valve, becomes undesirably long. This is a serious disadvantage when it is necessary to close a valve very rapidly so as to prevent the blowout of a well. This delay is due mainly to the fact that use is made of a flexible line which has the drawback of expanding as the pressure of the hydraulic fluid increases. It will be noted that the response time increases with the length of the lines.
To overcome this drawback, different solutions have been proposed. A first solution consists in using a battery of hydraulic fluid accumulators of higher capacity on the surface so as to obtain a large hydraulic fluid flow in the lines when the valve is opened or closed. It was thus hoped to reduce the control time. In fact, the lines used are generally of small diameter (about 4.8 mm). This results in significant pressure drops which limit and stabilize the hydraulic fluid flowrate in the lines.
It would also be possible to consider using rigid, and hence non-expanding, lines but one also comes up against a problem of pressure drops in the lines, and hence a limited flowrate, and the handling of the rigid piping is not at all practical.
Other solutions consist of using additional hydraulic fluid accumulators and placing them in the well, at the bottom, in the immediate vicinity of the valves to be controlled. In one of these solutions, the accumulators are controlled by means of hydraulic control valves actuated from the surface through hydraulic lines connecting these control valves to the surface. The opening or closing of these control valves is accomplished by varying the hydraulic pressure in the hydraulic control lines. It is then noted that the hydraulic lines are used only for controlling the main underwater valve, through the control valves, but not to furnish the hydraulic energy necessary for opening or closing the main underwater valve. In that system, the hydraulic control circuits and the hydraulic actuating circuits (i.e. furnishing the energy) are separate.
Another solution is described in the review "Offshore" of May 1979, pages 124-126. A battery of hydraulic fluid accumulators is also used in the well in the vicinity of the valves to be controlled. The accumulators are actuated by means of pyrotechnical valves triggered from the surface by means of an electric cable. This solution, while yielding remarkable performance, is complicated because it makes use of both hydraulic techniques and electrical techniques. Moreover, when the pyrotechnical valves have been triggered, they can no longer be used: the system will thus not operate repetitively. Generally, the use of accumulators placed in the well entails many drawbacks. They are in fact cumbersome and must be protected from shocks and from the fluids surrounding them. In addition, the pressure of the hydraulic fluid filling the accumulators must be adjusted from the surface taking into account the pressure prevailing in the well at the depth at which they are to be placed. This pressure adjustment calls for an auxiliary fluid source as well as a skilled operator. Similarly, when the accumulators are empty, it is necessary to bring the entire device up to the surface in order to recharge the accumulators. The device can thus be used for only a limited number of valve-actuation repetitions.